Wireless communications systems operate according to either a Time Division Duplexing (TDD) scheme or a Frequency Division Duplexing (FDD) scheme. For the FDD scheme, full duplex operation is typically desired, i.e., a radio transceiver is required to simultaneously transmit and receive in different frequency bands, which are referred to as a Transmit (TX) frequency band and a Receive (RX) frequency band, respectively. However, because the radio transmitter typically outputs signals at a power level that is much higher than that of desired signals received by the radio receiver, the radio receiver typically suffers from self-interference, or leakage, from the radio transmitter. In particular, a duplexer is often used to allow the radio transmitter and the radio receiver to share the same antenna. However, the duplexer provides limited isolation between the radio transmitter and the radio receiver. As such, transmitter leakage occurs from the radio transmitter to the radio receiver through the duplexer. Transmitter leakage may also occur even in implementations where the radio transmitter and the radio receiver are not directly connected (i.e., there is no duplexer and no antenna sharing between the radio transmitter and the radio receiver) or may also occur due to crosstalk leaking through integrated circuit substrates, device packages, or printed circuit boards.
Transmitter leakage decreases the sensitivity of the radio receiver. In particular, even though the transmitter leakage is in the TX frequency band, due to non-linear components in the radio receiver, the transmitter leakage can result in distortion in the RX frequency band. This distortion in the RX frequency band decreases the sensitivity of the radio receiver.
Techniques have been proposed that use active cancellation to cancel transmitter leakage. For example, U.S. Pat. No. 8,175,535 B2, entitled ACTIVE CANCELLATION OF TRANSMITTER LEAKAGE IN A WIRELESS TRANSCEIVER, which issued on May 8, 2012, describes systems and methods for active transmitter leakage cancellation. In one embodiment, a Radio Frequency (RF) cancellation signal is generated from a transmitter signal, and the RF cancellation signal is combined with a RF received signal to obtain a combined RF signal including residual transmitter leakage. The magnitude and phase of the RF cancellation signal are adjusted to reduce the residual transmitter leakage.
Similarly, in W. Schacherbauer et al., “An Interference Cancellation Technique for the Use in Multiband Software Radio Frontend Design,” 30th European Microwave Conference, October 2000, pages 1-4 (hereinafter the “Schacherbauer article”), the authors describe an active transmitter leakage cancellation scheme that utilizes an auxiliary transmit chain to generate an RF cancellation signal that is then combined with the RF receive signal to thereby cancel the transmitter leakage. The system disclosed in the Schacherbauer article can be illustrated as shown in FIG. 1. In particular, a wireless transceiver 10 includes a main transmitter 12 and a main receiver 14 that are connected to an antenna 16 via a duplexer 18. The duplexer 18 is also known as a duplex filter. The duplexer 18 includes a TX filter 20 that passes a RF transmit signal output by the main transmitter 12 to the antenna 16 and a RX filter 22 that passes a RF received signal from the antenna 16 to the main receiver 14. A Finite Impulse Response (FIR) filter 24 and an auxiliary transmitter 26 process the same input signal as that provided to the main transmitter 12 to generate a cancellation signal. A directional coupler 28 combines the cancellation signal and a RF receive signal output by the RX filter 22 of the duplexer 18 to provide a compensated RF receive signal in which the transmitter leakage has been reduced. The FIR 24 is designed to have a transfer function that models the transmitter chain components including antenna reflections.
The active cancellation scheme the Schacherbauer article relies on combines a cancellation signal with the RF receive signal to thereby cancel, or reduce, transmitter leakage. One issue that arises from these schemes is that a directional coupler has a coupling factor of, e.g., 10-20 Decibels (dB). As such, in order to overcome the coupling factor of the directional coupler 28, an output power of the auxiliary transmitter 26 must be larger than the actual leakage signal by the coupling factor of the directional coupler 28. As the output power of the auxiliary transmitter 26 increases, the power consumption of the wireless transceiver 10 also increases. Another issue that arises is that, due to, e.g., non-linear components in the auxiliary transmitter 26, the cancellation signal includes noise in the RX frequency band. This noise in the RX frequency band is injected into the main receiver 14, which can potentially degrade the sensitivity of the main receiver 14.
Thus, there is a need for systems and methods for active cancellation of transmitter leakage that reduces power consumption and/or reduces noise in the RX frequency band.